The present invention relates to a graphic display apparatus for performing antialiasing processing in order to display a three-dimensional figure on the CRT or printer without any staircases or jags, and more in particular to a graphic display apparatus in which rearrangement of pixels along the depth is made at write level by holding a plurality of pixel information for each depth.
Normally, oblique lines or polygonal sides of a figure develop staircases or jags when displayed on a raster-scan display such as a CRT. This is caused by the fact that when a figure is developed in pixels, pixels covered by part of an edge of a polygon are presented either in the color of the particular figure or in the color of pixels external to the figure. In order to solve this problem, a method has conventionally been adopted in which the color of a given pixel is determined from the proportion of the area that a polygon occupies of the pixel to the area of the pixel.
JP-A-5-290177, for example, discloses a method for determining the proportion that a polygon occupies of a pixel using the inclination of an edge of the polygon.
According to JP-A-6-162209, on the other hand, there is disclosed a method involving a straight line and a rectangle, comprising a memory means equivalent to a frame buffer for holding an antialiased graphic image, a memory means equivalent to a Z buffer for holding a Z-value information for three-dimensional processing, and another memory means for storing a graphic image not antialiased, wherein in the case where another figure is plotted by a three-dimensional graphic plotter behind a figure once antialiased, the overlapped portion can be antialiased again. The description in this publication is not clear and contains inaccurate teachings as long as the present inventors understand them. Since at least connections of adjacent polygons are not taken into consideration, however, it seems that when a curved surface is displayed approximately by small triangles, the pixels between triangles cannot be accurately determined.
JP-A-4-233088 (corresponding to GB pat. Appln. Nos. 9014528.5 and 9014555.8 filed Jun. 29, 1990) is a disclosure of a method in which a figure is developed in pixels in such a way that several images with slightly displaced origins are synthesized to blend the colors of the pixels in the proportion of area of the images that represents each pixel. This method, in which a plurality of images are required to be generated and synthesized, takes a correspondingly longer processing time.